This invention relates to a semiconductor switched bridge circuit which minimizes the number of high voltage components through feedback based inherent parasitic capacitance and which controls circuit output by differentiating between hard and soft switching.
High voltage half-bridge circuits are used in many power applications including D.C. to A.C. converters, motion control devices, switch mode power supplies, power motors, and lighting ballasts. A high side switch and a low side switch selectively couple a source voltage to an output node VHB. These switched circuits control high side switching. Ideally, the output of these circuits should be a high frequency, high voltage square wave feeding at least a partially reactive load.
The output voltage VHB can be partially driven, in steady-state operation, by oscillation generated by reactive elements within the load. When the low side switch conducts, the inductor in the load conducts as well. When the low side switch is turned off, the inductor tries to keep current flowing by raising the voltage at node VHB up to the DC rail. This is called "soft" switching.
The current flowing through the inductor may not be large enough to raise the VHB voltage to the D.C. rail due, in part, to the counteracting capacitive affect from both parasitic and discrete capacitances. Once the voltage at node VHB stops increasing (dVHB/dt=0), it is desirable to turn the high side switch on (i.e. referred to as "hard switching") to continue the increase. Such switching is necessary during initial start-up of the circuit and when power has been dissipated in the load. Conversely, when the high side switch is turned off while there is still current running through it, the inductor will try to bring the voltage at node VHB to the ground rail. Hard switching may be necessary to reach the ground rail.
Unfortunately, hard switching dissipates power. To minimize the power dissipated, the high side switch should be turned on when the voltage at node VHB is at a high level caused by power being fed from the partially inductive load. At this point, there is a low voltage drop and thus minimal power dissipation. Clearly, zero voltage switching, that is when the voltage at node VHB is at the D.C. rail, is not possible during start up because there is no energy in the inductive load to raise the voltage VHB to the rail voltage. Therefore, at least during start-up periods, hard-switching is required.
In U.S. Pat. No. 5,068,571 a high voltage capacitor is used to detect the transition periods of the output. A transition is sensed in this capacitor when the current through it (Cdv/dt) is not equal to zero. This is output to amplifiers which keep both switches off during any transition in the output.
This solves the problem of a false turn-on due to parasitic currents thereby making the circuit lower power and thus more integrable. However, it also prevents the high side switch from being able to turn on during a hard switching condition. Such hard switching is commonly necessary in many power converter applications (including zero voltage switching applications) as is stated above. Hard switching is especially needed during the initial powering of the device. U.S. Pat. No. 5,068,571, which does not allow hard switching, would hinder the powering up of the circuit. Additionally, it requires a dedicated high voltage capacitor which again increases the number of high voltage components thus defeating the area saving benefit of a single-ended solution.
Therefore, there exists a need for a circuit which can control the high and low side switches in response to whether hard or soft switching is desired and which limits the number of high voltage components.